The invention relates to a device for producing an electric signal which corresponds to the position of an object which is movable in rotation or translation relative to a base. Analog voltage signals A, A and B, B are produced by an optical scanning of periodically recurring markings upon movement of the object. The voltage signals are obtained by means of two sensors which are arranged apart from each other by one quarter of the scanning period, the signals being converted into digital signals A and B (quadrature signal) in an evaluation circuit (11) having comparators (12, 13).
In such position indicator devices, the position of a movable object relative to a base is represented by a count value. A reversibly unequivocal association between position and count value exists in the long run, however, only if further conditions are satisfied. Thus, the last count value upon a placing out of operation must be stored in permanent fashion and, furthermore, assurance must be had that the object cannot change its position when the counter is disconnected. As an alternative, it can also be provided that, upon a placing in operation, one always starts from a given position and a corresponding count value. However, this is not always possible, or involves considerable expense since, within the scope of a learning routine, first of all the intended position must be moved too, and automatically ascertained and the counter set to the starting value before the actual placing in operation. Furthermore, counting errors which occur during the operation cannot be compensated for by this.
It has therefore already been proposed to provide, in addition to the periodically recurring markings, an index marking with which, over the entire setting range, a plurality of absolute values of the position can be determined or with which incremental addition and subtraction can be checked and possibly corrected at specific support places (see Federal Republic of Germany 40 41 491 A, and 42 17 168 A1).
With none of the known measures, however, can it be determined whether the periodically recurring markings or the index markings lose their distinguishing force with time. Thus, for instance, in the case of the widely used marking by light-transmitting slots alternating with light-opaque bars, the slots become entirely or partly clogged by deposits produced by wear, so that the light signals become weaker or are even not obtained at all. In this way, one or more counting pulses may be lost so that the association between position and count value is inaccurate. The same error can occur when the source of light used becomes weaker or breaks down temporarily or permanently.